Honestly I have no idea about the tax information in canada, maybe someone else from around there might know.
As far as what we get back. A medical deduction takes away from the adjusted gross income so the tax liability gets lowered. As a result, someone who makes a LOT of money may still owe some federal taxes (hopefully paid them throughout the year via deductions though) but it might be signifigantly less (depending upon income and amout of deductions). Some families who make a little or are right in the middle, may end up having NO tax liablity at the end of the year.
There are also child tax CREDITS and earned income CREDITS and those work differently than medical deductions. Whereas with deductions you can only get a refund in the amount you paid in federal taxes (so if you paid nothing, and your tax liablity ws nothing, you would get nothing back BUT if you paid $2500 and your tax liablity was nothing you would get $2500 back), with one of the CREDITS you can get back more than you paid. It's exactly what it is, a credit. For exmaple, someons income is $30,000 they have 3 kids and $13,000 in medical expenses. The medical deduction would help them, take their tax liablity down to nothing. But they also have 3 kids so they would qualify for an earned income credit and a child tax credit. So, not only could they get back what they paid in taxes, but they could get MUCH MORE.
Hope that explains it.
